Influence of metal ions on thermal aggregation processes of globular proteins.

Metal ions are implicated in protein aggregation processes of several neurodegenerative pathologies, where the protein deposition occurs. The idea arises from the finding that increased metal concentrations (mainly copper, iron and zinc) are present in the brains of Alzheimer's disease patients both in the amyloid plaques and in the cortical tissue. Cu2+ and Zn2+ metals ions have a different physiological role and it has been observed that, in vitro, both promote (zinc more than copper) aggregation in amyloid fibrils and/or in amorphous aggregates. In addition, studying aggregation protein process can be of interest also in the field of biotechnologies like the food technology, since most processes in manufacturing of foods are based on thermal treatments. Knowing the structures of the proteins during and after these treatments is extremely important also for allergenic problems. The effects of Cu(II) and Zn(II) ions on heat-induced structural modifications of bovine serum albumin (BSA) and beta-lactoglobulin (BLG) were studied, with the aim of delineating the role of these ions in the early stages of proteins aggregation kinetics. A joint application of different techniques was used. The aggregate growth was followed by Dynamic Light Scattering measurements, whereas the conformational changes occurring in the protein structure were monitored by Raman and IR spectroscopy. Both in absence and in presence of metal ions, heating treatment gave rise to p-structures to the detriment of oc-helix conformation of the proteins. Different efficacy of the Cu(II) and Zn(II) ions in promoting the protein aggregation were highlighted by Raman measurements, assessing the role of His residues in metal binding. A distinct polypeptide folding of the two metal-protein systems takes piace, since the predominant mode of metal binding depends on metal.